Machines such as motors and generators typically include a rotor and a stator. The rotor may include a permanent magnet that produces a magnetic field. In a generator, rotation of the rotor with respect to the stator causes the magnetic field from the permanent magnet to rotate, thereby inducing electric current flow through windings in the stator. Conversely, in a motor, current flow through the stator windings induces rotation of the permanent magnet, and thus the rotor.
Operation of the machine creates losses and then subsequent heating in the permanent magnet. For a given operating point and design, the permanent magnet has an effective maximum allowable operating temperature, which, if exceeded by even a few degrees for a short period of time, may result in sudden and irreversible demagnetization of the permanent magnet which prevents the machine from operating at its designed potential. This risk can be particularly acute during the event of a sudden short circuit fault of the generator or seen by the generator, for example, a short circuit in the windings, terminals, or cabling, or more commonly, of the generator control electronics such as a rectifier or power electronic converter. As a result, to mitigate this risk, machines are often designed with larger and higher-grade (e.g., higher allowable temperature) permanent magnets and/or operated at reduced loads to reduce the risk of demagnetization of the permanent magnet. In addition, machines often include control systems that monitor the temperature of the permanent magnet to prevent excessive temperatures.
Directly monitoring the permanent magnet temperature is not feasible because of the rotation of the rotor. Therefore, control systems are typically limited to, at best, indirectly monitoring the permanent magnet temperature by monitoring the temperature of the stator windings. For example, resistance temperature detectors (RTDs), whose resistance varies proportional to the temperature, may be physically installed within the stator. Wiring from the RTDs transmits a temperature signal from the RTDs to the control system, and the control system may use the temperature signal to adjust the load of the machine to reduce the temperature of the stator, and thus hopefully also the temperature of the permanent magnet. This arrangement, however, is also susceptible to failure of the RTDs or wiring. As a result, multiple RTDs and associated wiring are typically installed within the stator to provide redundancy. Furthermore, with only stator winding temperature detection, there is no direct knowledge of the magnet temperature or guarantee that the magnet temperature is within safe or desired bounds. Therefore, controlling the load on the machine based on the stator temperature may unnecessarily limit the rated load of the machine or put the magnets at unacceptable risk of irreversible demagnetization.
Therefore the need exists for a system and method for determining the temperature of the permanent magnet in machines. Ideally, the system and method will reliably provide an accurate reflection of the permanent magnet temperature without requiring redundant sensors and without unnecessarily restricting the rated load of the machine.